1. Field of the Invention
The present invention relates to a semiconductor wafer accounting and processing system which can detect both the number of wafers stored in a wafer cassette and their positions therein.
2. Description of the related art including information disclosed under .sctn..sctn. 1.97-1.99.
During the manufacturing of semiconductor integrated circuits, a wafer detector is used to monitor the presence/absence of wafers in units of lots and to check the number of wafers for supplying to the next process. This the wafer detector does by detecting the number of wafers stored in the wafer cassette and their storage positions.
Such a wafer processing system disclosed in, for example, Japanese Utility Model Disclosure No. 61-129340. FIG. 1 of this disclosure shows a method of detecting the number and positions of wafers, using this wafer detector. Referring to FIG. 1, reference numeral 1 denotes wafers stored in a wafer cassette (not shown). Light-emitting element 3 and light-receiving element 4 are arranged surrounding the wafer cassette and opposing each other such that optical axis 2 is inclined at predetermined angle 8 with respect to a surface of wafer 1. Light-emitting and light-receiving elements 3 and 4 intermittently move along a direction formed by the arrangement of wafers 1. This moving pitch of these elements 3 and 4 is equal to an arranging pitch D of wafers 1. When infrared rays are emitted from light-emitting element 3 toward light-receiving element 4, the infrared rays along optical axis 2, are shielded by wafer 1 and the amount of infrared rays received by light-receiving element 4 is attenuated. As a result, the presence of wafer 1 is detected by this attenuation of infrared rays. On the other hand, when the amount of infrared rays is not attenuated, the absence of wafer 1 is confirmed. The presence/absence of wafer 1 is detected in each intermittent movement of light-emitting and light-receiving elements 3 and 4, thereby detecting the number of wafers 1.
According to another wafer detector, a reflecting-type sensor is used. Infrared rays are radiated from above a wafer cassette onto a wafer and the reflected rays are received by the sensor so as to detect the presence/absence of the wafer. Similarly, in this case, the number of wafers is detected by emitting infrared rays and receiving reflected light while intermittently scanning the light-emitting element and the light-receiving element along an arranging direction of the wafers.
However, in such conventional wafer detector, emitted infrared rays may be irregularly reflected from a surface or end portion of a wafer since the light is not always radiated perpendicular to the surface or the end portion and sometimes dust and fault exist there. In this case, the light-receiving element erroneously detects the wafer. As a result, the positions and number of wafers cannot be accurately detected.